The growth factor receptors c-kit, erythroprotein receptor (EpoR), and insulin-like growth factor 1 (IGF-1R) are found on erythroid progenitor cells and play important roles in erythropoiesis. Recent work has shown that the addition of c-kit ligand to IL-3 dependent 32 D cells expressing only both c-kit and EpoR is able to activate the erythropoietin receptor signaling pathway and mediate cell proliferation, suggesting a novel mechanism by which a ligand may transmit an intercellular signal through a heterologous receptor. Specifically, this proposal will further characterize this signaling pathway as well as determine whether this mechanism is utilized in the development of other cell lineages and in the etiology of disease. more broadly, this research will increase our understanding of signal transduction mechanisms involved in hematopoiesis as well as provide potential targets for the treatment of hematopoietic disorders. Utilizing EpoR mutants, this proposal will first determine whether c-kit mutations also affect megakaryocyte and mast cell development. Experiments will examine whether c-kit is able to interact with and activate the thrombopoietin growth factor receptor c-mpl. Also, a yeast two-hybrid system will be utilized to identify novel receptors interacting with c-kit in mast cells. Finally, the receptor tyrosine kinase of IGF-1 which is implicated ina the etiology of the myeloproliferative disorder polycythemia vera (PV), will be assayed for its ability to interact with and activate EpoR. IGF-1 receptors are found on all hematopoietic cell types, yet the phenotype of this disorder is largely erythroid specific.